NAME

SYNOPSIS

DESCRIPTION

The kernel keeps for each process two values called
set_child_tid
and
clear_child_tid
that are NULL by default.

set_child_tid

If a process is started using
clone(2)
with the
CLONE_CHILD_SETTID
flag,
set_child_tid
is set to
child_tidptr,
the fifth parameter of that system call.

When
set_child_tid
is set, the very first thing the new process does
is writing its PID at this address.

clear_child_tid

If a process is started using
clone(2)
with the
CLONE_CHILD_CLEARTID
flag,
clear_child_tid
is set to
child_tidptr,
the fifth parameter of that system call.

The system call
set_tid_address()
sets the
clear_child_tid
value for the calling process to
tidptr.

When
clear_child_tid
is set, and the process exits, and the process was sharing memory
with other processes or threads, then 0 is written at this address,
and a
futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL, 0);
call is done.
(That is, wake a single process waiting on this futex.)
Errors are ignored.

RETURN VALUE

set_tid_address()
always returns the PID of the calling process.

ERRORS

set_tid_address()
always succeeds.

VERSIONS

This call is present since Linux 2.5.48.
Details as given here are valid since Linux 2.5.49.